The development of cadmium sulfide (CdS)--cuprous sulfide (Cu.sub.x S) photovoltaic cells comprises a rapidly developing technology. The component layers may be formed by vacuum deposition, spray and dipping processes which are adaptable to volume production of large area photovoltaic panels.
A completed photovoltaic panel is formed from a plurality of thin films superposed on a common substrate. The selected substrate generally has at least one electrically conductive surface and may be transparent.
A first semiconductor material is formed over the substrate by techniques well known in the art such as vacuum deposition, sputtering, sintering, or spraying. In the present invention, the semiconductor may be CdS or a solid solution of CdS and another sulfide, such as ZnS.
Next, cuprous sulfide is formed over the semiconductor layer and a heterojunction obtained between the cuprous sulfide and the semiconductor. Finally, a conductive layer is deposited over the heterojunction to form the positive electrode.
It is well known that the stoichiometry of the Cu.sub.x S layer is an important factor in the energy conversion efficiency of the heterojunction. Cu.sub.x S occurs in several phases such as Cu.sub.2.00 S (chalcocite), Cu.sub.1.95 S (djurleite), and Cu.sub.1.80 S (digenite). Stoichiometric Cu.sub.2 S (chalcocite) is required for high efficiency photovoltaic cells. It has been found that efficiency decreases by 40% for djurleite and 90% for digenite.
A conventional technique for forming the Cu.sub.x S layer uses ion exchange to replace cadmium with copper through the reaction: EQU CdS+2Cu.sup.+ .fwdarw.Cu.sub.2 S+Cd.sup.2+
The reaction may be accomplished by immersing the layer of CdS in a cuprous dip such as described in U.S. Pat. Nos. 3,374,108 and 3,416,956 to Keramidas.
The actual stoichiometry of the Cu.sub.x S layer formed from the cuprous solution is not equal to the theoretical value X=2. Oxidation of cuprous ions in the solution and of the resulting cuprous sulfide layer result in X&lt;2. Prior art attempts to improve the stoichiometry have frequently produced metallic copper in the photovoltaic layers. The presence of metallic copper acts to degrade cell performance and negates gains produced by the improved stoichiometry.
Yet another cause of metallic copper diffused into the photovoltaic layers is the use of copper as the positive electrode. Upon completing the photovoltaic cell, it is generally necessary to heat treat the cell to complete formation of the heterojunction. Metallic copper from the electrode readily diffuses into the cell during this heat treatment and produces the undesirable degradation in cell performance.
Copper has many other desirable characteristics for CdS--Cu.sub.x S photovoltaic cell application including cost, ease of application, and material compatibility. The disadvantages of the prior art are overcome by the present invention, however, and improved methods are provided for manufacturing CdS--Cu.sub.x S photovoltaic cells having copper positive electrodes.